A Sea breeze typically occurs during summer along the coast line of the ocean or a lake.
If you sail at the same spot during the summer, some days the sea breeze is steady and consistent. This onshore wind fills in almost like clock work and it is a guarantee for a good day on the water. But the next day, the sea breeze may be much weaker or not come at all, and you just end up waiting for the wind all afternoon. Finally on another day the sea breeze may come much stronger than usual changing an easy afternoon sail into a challenging one.
This article will dive deep into the sea breeze to bring you knowledge and tools to apprehend this phenomenon at your local spot or any place where you intend to sail.
This article will cover:
I. Sea breeze theoretical model
II. Coriolis force
III. Coast line and topography
IV. Atmospheric stability
V. Gradient wind
VI. Cloud cover and rain
VII. Night breeze
I. Sea breeze theoretical model
Here we will discuss the sea breeze occurring during the day, more relevant for most sailors than the night sea breeze.
Let's start with a simple coastline that is straight and flat with no topography. Let's assume that there is no wind at all in the early morning, meaning there is no gradient wind driven by large scale weather patterns.
During this nice summer day as the sun rises, here what will happen:
the air temperature over land rises higher than over water (due to different thermal capacity between land mass and water)
this greater heating of the air over land results in the expansion of the air column. It makes a small low pressure on the ground, compared to a higher pressure over the surface of the water resulting in an onshore flow at the coastline. This onshore cooler flow then acts as a lever to increase the rise of the hot air inland.
As the air rises over land, it cools down at higher altitude around 1 kilometer high, increasing its density and forming a high pressure zone aloft. This cooler and denser air flows back toward the ocean aloft, completing the circulation loop.
source: NOAA
Typically the sea breeze:
extends 10 to 100km horizontally
is around 1 km deep in altitude.
blows over the water around 10 to 20 knots.
Here is what a sailor experiences on a typical sea breeze day:
8-10am | no/calm wind |
10-11am | the breeze starts to fill in slowly. It is patchy usually close to shore |
12pm-2pm | wind slowly builds from the shore and then extends offshore |
2-4pm | wind reaches its maximum speed in early afternoon (when land is the warmest) and holds for a bit. It normally turns right in the North hemisphere (left in the South) |
4-5pm | wind quickly decreases and dies out |
This theoretical model is important to know. But in reality many factors influence the sea breeze. The rest of this article will discuss some of those factors.
II. Coriolis effect
The Coriolis effect (introduced in a previous article Marine Meteorology 2: Wind) has a greater deflection the longer the air particle is in motion. So you need a large distance and a long time to notice the Coriolis effect in meteorology. We consider that 100 kilometers and 3 hours are the criteria for the Coriolis component to be relevant.
Sea breeze phenomena is right at the edge in terms of distance and time scale.
So for a large sea breeze extending more than 100 km offshore and lasting for more than 3 hours, we can expect the sea breeze to turn right with time in the Northern hemisphere, and left in the Southern hemisphere.
III. Coast line and topography
Coastline Shape: The shape of the coastline can influence the direction and strength of the sea breeze. For example, concave coastlines may focus the breeze, while convex coastlines may disperse it.
Topography: Mountains or hills near the coast can enhance or disrupt sea breezes by blocking or channeling the airflow. Lakes near mountains are particularly impacted (eg. Garda lake in Italy).
Some regions of the world are characterized with a cold ocean next to a coastline that gets extremely hot during the day. Those create perfect places for strong and consistent sea breeze to develop.
As an example the South of Morocco is characterized by a cold Atlantic ocean in contrast to the extreme heat of the Moroccan desert falling in the sea. In Western Australia (Perth) the relatively cold Indian ocean meets the extreme heat from the outback.
IV. Atmospheric stability
If the atmosphere is very stable in the morning, it will fight against the warm air heated by the sun trying to rise. Therefore the sea breeze may not develop well. Ideally the air needs to be unstable so it will supports the rising air and help the sea breeze to develop.
A very stable atmosphere is present when there is for example a temperature inversion. The temperature inversion happens when the air temperature is warmer in altitude than below (typically the air temperature decreases with altitude). A visual clue of a temperature inversion is when rising smoke, from a factory or a fire, is blocked in altitude by the temperature inversion that acts like a glass ceiling prohibiting the rising smoke to keep rising and therefore extending horizontally at the height of the temperature inversion, see image below:
On the other hand, if there is some instability in the atmosphere, as soon as the air warms up over land due to the sun heating, the air will rise and will be accelerated up by the instability that wants to lift it further rather than bringing it down. The upward motion over land is then established and the sea breeze can now build up to make a great sailing day.
V. Gradient wind effect
In part I, we presented the sea breeze theoretical model and we assumed that there was no wind in the morning prior to the sea breeze establishing. But the reality is most days there is always a "background" wind due to a large scale weather pattern. This wind is referred as the gradient wind or synoptic wind.
The gradient wind present in the morning can help jump start the breeze by creating favourable conditions so the sea breeze can build upon. Below we will present how gradient wind strength and direction can both play a role. Of course this is theoretical, and the reality on any give day may be a bit different. Therefore the reader must take this with a pinch of salt and use the theory to get a grasp of the situation and not follow it blindly.
The upper return flow of the sea breeze is around 1 kilometre high in altitude (900 mbar). Having some offshore gradient wind flow in the morning can help the sea breeze to develop. This altitude offshore wind will jump start the sea breeze by already having the return flow present aloft. However it is important that this gradient offshore flow is not too strong, especially at the surface otherwise the onshore sea breeze will fight against it and may not develop at all.
So a weak or moderate offshore wind in the morning is a good first criteria.
The second criteria is the direction of the gradient wind relative to the coastline. The angle between the coastline and the wind direction impacts the stability of the air at the coastline and therefore its capacity to rise (see part IV above Atmospheric stability).
As the surface the wind blows from land to the water, and it will experience less friction as it crosses the shoreline.
if the wind is perpendicular to the coastline, then the wind will uniformly accelerate.
if the wind is angled relative to the coastline, it will create some air particle vorticity at the coastline. The diagram below shows that the air particle at the coastline will experience one side to accelerate by one knot, thus making the particle to spin.
the vorticity can be positive, making the air unstable and likely to rise, thus creating favourable sea breeze conditions
the vorticity can be negative, making the air stable and unlikely to ruse, thus making unfavourable sea breeze conditions.
So to summarize, here are the favourable conditions for the offshore gradient wind in the morning for the sea breeze to develop.
If you are in the NORTHERN atmosphere and you are on the coast looking inland perpendicular to the coast, you want the wind to come from your LEFT at an angle.
If you are in the SOUTHERN atmosphere and you are on the coast looking inland perpendicular to the coast, you want the wind to come from your RIGHT at an angle.
VI. Cloud cover and Rain
The cloud cover over land will have an impact on how much the land will warm during the day. A thick cloud cover in the morning will prevent the heating and may completely prohibit the sea breeze from developing. Inversely, cloud cover over the water and not over land may allow a stronger differential heating between land and water resulting in a stronger sea breeze.
When there is heating over land, clouds will start to develop over land due to the rising of the air over land. Cloud development on land, especially puffy cumulus, are a good sign of a nice sea breeze developing. See image below of a visual sign of a good sea breeze developing
source: https://local-news-archive.crystalbeach.com/recreational-boating-safety-rain-and-wind-and-fire/
As the afternoon develops some of those clouds may develop into cumulonimbus and start to precipitate over land. The precipitation creates a downward air flow which will prohibit the the rising of the air overland and therefore may kill the sea breeze quickly and the sailor will feel like someone as turned off the "switch" of the sea breeze.
But the clouds over land may be pushed over the water by the altitude flow of the sea breeze. If the clouds precipitates over the water, then the downward motion of the air over the water may actually boost the sea breeze.
VII. Night sea breeze
At night, it is the opposite. The air temperature over land will cool more than over water and the sea breeze will have the opposite direction (offshore flow).
source: NOAA